1. Field of the Invention
The current invention relates to a flame photometer detector having improved sensitivity.
2. Discussion of Prior Art
The analytical technique of flame photometry is well known. Typically, a sample is introduced to a hydrogen flame and electrons in the outer shell of atoms of the substance of interest are excited to a higher state of energy. When the electron returns to its ground state, energy is emitted in the form of light by which the presence of the substance is confirmed.
The wavelength of the emitted light depends on the substance of interest. Typically the light is detected using a photomultiplier tube (PMT) which provides a quantitative analysis of sample present. An optical filter having a narrow passband centred on the wavelength associated with the substance affords selectivity.
One application in which flame photometers are used is the testing of military respirators.
The effectiveness of the respirator in providing protection against a particular agent is expressed as the protection factor (PF). This is the ratio of the concentration of agent outside the respirator to that found within. The concentration within results from a combination of the various leak paths, namely the face seal, the outlet valve and the filter.
One method of testing respirator leakage involves measuring the penetration of solid salt (sodium chloride) aerosol using flame emission spectroscopy as the detection method. A subject wearing a respirator under test is placed in a test chamber into which the aerosol (typically of concentration 13 mg mxe2x88x923) is introduced. The aerosol is produced using a Collison atomiser to aspirate a salt solution into droplets. These are diluted to the required concentration and dried using a fan to produce the solid aerosol which enters the test chamber. The solid aerosol so produced has a mean diameter 0.6xc3x9710xe2x88x926 m but the particle sizes range from 0.05xc3x9710xe2x88x926 m to 1.2xc3x9710xe2x88x926 m. During testing a continuous sample is drawn from inside the face mask of the respirator into the flame chamber of the photometer.
On entering the flame, the salt gives rise to radiation having a wavelength of 598.6 nm. This passes through a narrow pass filter, which inhibits transmission of other light. When measuring high salt concentrations, neutral density (ND) filters are introduced to prevent flooding of the photomultiplier. These filters reduce the intensity of light reaching the photomultiplier by a calibrated amount.
The response from the photomultiplier is recorded on, for example, a chart recorder and the effect of any ND filters is taken into account in the following expression for calculating salt concentration:
total deflection due to salt=antilog ND value (salt signal)xe2x88x92clean air signal
This deflection value is then used to obtain the percentage penetration calculated from a calibration curve produced specifically for that instrument. A penetration of 0.01% is equivalent to a PF of 104.
A typical limit of detection of devices currently in use is about 20 ng mxe2x88x923. For the test procedure described above, this corresponds to a PF of 6.5xc3x97105.
The current requirement for the PF of high efficiency military respirators is 104 but this likely to be significantly increased, due to the increased demands of the modern battlefield environment.
Thus, in addition to the general benefit to the art of flame photometry afforded by a device which offers improved detection limits, there is a specific requirement, for the testing of high efficiency military respirators.
Another problem encountered during the testing of respirators is that of lung retention: continuously sampling air from the inside of the mask (during both inhalation and exhalation) means that observed results are affected by retention of salt by the lungs.
At present, this is accounted for by exposing the challenge aerosol to lung retention at the end of the respirator test and deriving a lung retention factor which is incorporated in the calculation of percentage penetration. This method is not seen as ideal because it may not be appropriate to use the lung retention factor measured for the neat cloud for calculations involving exposure to low concentrations of salt inside the respirator during test. A typical analysis from the output of a Collison atomiser (based on a 2% w/v solution of NaCl in water and a 12.5 Lminxe2x88x921 flow through the saline solution diluted to 90 Lminxe2x88x921 with atmospheric air) shows a solid NaCl concentration of xcx9c13 mgmxe2x88x923. This enables an estimate of the detection limits required at a PF of 106.
According to the present invention a flame photometer detector comprises:
a flame chamber having a burner, a hydrogen inlet, a sample inlet and a window, transparent to radiation produced therein;
means for detecting independently a first radiation and a second radiation, and producing corresponding electrical signals, wherein the intensity of the first radiation is dependent on the concentration in the flame chamber of at least one chemical species to be detected and of one or more background chemical species and the intensity of the second radiation is substantially independent of the concentration in the flame chamber of the chemical species to be detected and is dependent on the concentration in the flame chamber of the background chemicals species and
data acquisition means for measuring the electrical signals and comparing the data so obtained with calibration data obtained in the presence of the background chemical species and the absence of the chemical species to be detected.
Preferably the data acquisition means is capable of acquiring data at a rate sufficient to record single particle events within the flame chamber.
A further preferred embodiment includes means for modulating the first and second radiation, the detecting means being selective to radiation so modulated.
A further preferred embodiment includes means for directing radiation exiting the flame chamber on to the filters and, or means for directing modulated radiation towards the means for detecting radiation. Such means for directing radiation might comprise a lens.
The detecting means comprises a photomultiplier tube.
In a particular embodiment the detector is used in testing the efficiency of respirators, and further comprising means for directing sample air from the interior of a respirator to the flame chamber during inhalation by a subject wearing said facemask. Means might also be included for directing fresh air to the interior of the respirator during exhalation.
According to a second aspect of the invention, a method of measuring the presence of a chemical species using a flame photometer detector includses the steps of:
i) measuring a first radiation and a second radiation, and producing corresponding electrical signals, wherein the intensity of the first radiation is dependent on the concentration in the flame chamber of the chemical species to be detected and of one or more background chemical species and the intensity of the second radiation is substantially independent of the concentration in the flame chamber of the chemical species to be detected and is dependent on the concentration in the flame chamber of the background chemicals species and
ii) comparing the data so obtained with calibration data obtained in the presence of the background chemical species and the absence of the chemical species to be detected.